1. Field of the Invention
This invention pertains to data communications and particularly to data communications on a wireline such as one employed in an oil or gas wellbore application.
2. Description of the Prior Art
It is common in an oil or gas wellbore application to transmit and receive electrical digital and data control signals between surface electronics and a downhole electronics package via a wireline of one or more conductors connecting the two. Such signals are typically used to remotely control the functions of various downhole devices such as sensors for detecting borehole parameters as well as tools and devices for performing functional operations in the borehole such as setting equipment or operating testers, motors, directional drilling equipment or the like, which may be operable in stages and in any event requiring a plurality of differing control signals at different times. Likewise, it is desirable to transmit information indicative of the operation of downhole devices or parameters detected or measured downhole, to the surface over the same conductor path. It is customary in such downhole operations to utilize a sheathed or armored cable which includes a single conductor as a core insulated from a protective conductive sheathing, which also acts as another electrical circuit path in conjunction with the core conductor to provide a conductive pair. Such so called single conductor wireline cables, or similarly constructed multi-conductor cables, are almost exclusively used to operate downhole electrical devices because of a variety of reasons associated with the space limited and rigorous environment of a wellbore. In such oil and gas wellbore operations, a wellbore depth of many thousands of feet is not uncommon. In communicating between the surface and downhole in a wellbore over a wireline cable, control signals and data signals are normally converted to a digital code comprising a plurality of "0" and "1" bits that are transmitted at rates up to a maximum of 4 Kbits/second. A "1" is typically represented by a voltage sequence. That is, the "1" and "0" bits are represented by a sequencing of voltage levels. A "1" bit could be represented by a single first voltage level (e.g., a relatively high level) and a "0" bit could be represented by a single second level (e.g., a relatively low level). In the non-return-to-zero (NRZ) format, a "0" digit is commonly represented by a predetermined lower level voltage which may or may not be zero volts. A "1" is represented by a higher predetermined voltage level. Each bit has a predetermined time interval associated with it. Two or more successive bits of the same kind, either "0" or "1", is represented by no change of voltage. There is only a voltage change when there is a change from a "0" to a "1" or a "1" to a "0". It is understood that there are other modulation schemes in common use, such as bi-phase voltage sequences and delayed modulation sequences, which are more complex than NRZ. However, the problems imposed by the wireline as discussed herein affect them all.
Continuing using NRZ as an example, a coded digital word would appear as a variable period, two-level rectangular wave voltage varying between a first voltage level and a second voltage level. The control and data information is carried by the changing voltage levels and by the number of bit time periods between the occurrences of the voltage changes. Hence, a conventional receiver or detector detects the first and second levels and the times of occurrence so as to be able to decode the transmission. As mentioned previously, the transmission and receiver scheme just discussed operates well when the rate of transmission does not exceed about 4 Kbits/second or the wireline is relatively short.
However, the wireline transmission medium does cause a problem when the transmission is over a relatively long length or as the data rate increases. That is, the detection and distinguishing of the two voltage levels is impaired by distortions caused by the medium. Distortions become more acute for faster bit rates, where the periods at each of the two voltage levels are very short. For example, the frequency characteristic of a typical single conductor wireline used for downhole application has about a 3 db loss at 5.6 Khz for a 30,000 foot length. At higher frequencies, the loss is significantly greater. When the loss reaches this 3 db level, it is referred to as a "cut off" or "roll off" frequency.
Cut off is measured by increasing the frequency of a signal over a medium until the signal falls off or is attenuated to one half its transmitted amplitude due to losses in the medium. In the present data transmission system, a 9.6 KBaud data rate is being used. Ordinarily, good data transmission design practice would require a transmission medium having a cut off frequency of at least 11/2 times the data rate being used. This would dictate a transmission medium having a 14.4 Khz cut off frequency whereas the best low loss wireline in common usage in oil field work has a 5.6 Khz cut off, such wirelines being designed primarily for their mechanical capabilities as opposed to high frequency transmission characteristics, to accommodate the physically hostile borehole environment.
Distortion consists primarily of amplitude losses and phase error. It is possible to overcome amplitude losses by making the voltage level between the two bit types be greater than for shorter line transmission. For example, a typical voltage level for a "0" bit could be 0 volts and a typical voltage level for a "1" bit could be 30 volts, a 30-volt difference. This difference could be doubled or made even greater so as to increase transmission efficiency for a longer transmission distance. However, there are practical limits as to what the voltage differences can be, particularly in the presence of a higher rate of transmission, such as 16 Kbits/second.
An even more significant source of error, as the wireline length and/or data rates increase, is the phase errors of the received pulses. Phase error in this case describes the time distortion of a pulse by the transmitting medium so as to change the relative position of the pulses in a data stream from that of the original data stream. Significant phase error can make the time position of a pulse ambiguous and result in a data error. The amount of phase distortion incurred is proportional to the characteristics of the medium, transmission rate, modulation scheme and the particular data being sent at a particular time. Phase error is the principal limitation to high speed data transmission over wireline, limiting normal operation to less than approximately 4,000 bits per second.
In developing the process, it was observed that the instantaneous magnitude of phase distortion in a wireline being operated significantly above cut-off frequency is directly related to the rate of change of the average level of signal present on the wireline. This change occurs as a result of the data signal being impressed on the wireline. All data streams contain a DC or average component associated with them which is data dependent. A string of "ones" has a different level than a string of "zeros" and a string of ones and zeros, which would be typical of data, will have something in between. This average component of the data pushes and pulls the average signal level on the wireline up and down as the data changes and phase distortion results.
The amount of phase distortion present at any instant of time is dependent on the data being sent, the data coding scheme used, and how high the operating frequency is above the cut-off frequency. Oil field cable, whether single or multi conductor, has characteristics which shift the leading and trailing edges of a signal in the time domain to generate phase error. If the occurrence of a leading or trailing edge is shifted forward or backward greater than one-half a bit time, it introduces an ambiguity into the data recovery process and there will be some bits that cannot be uniquely determined as to whether they are a one or a zero. This can be resolved to a certain degree by using error correcting codes or by establishing certain conditions into the data that the receiving circuitry can examine to decode ambiguities, but any such scheme complicates the data recovery process and can significantly increase the overall complexity of the receiver.
Other than using error correcting codes, algorithms or other schemes for manipulating data received to eliminate transmission induced ambiguities, little has been done to correct this particular problem associated with wireline cable in the borehole environment in a more simple and less complex manner.
Therefore, it is an object of the present invention to provide an improved method of transmitting digital data in a borehole data transmission system between the surface and downhole at data rates above wireline roll-off frequency.
It is another object of the present invention to provide an improved method of transmitting digital signals in a wellbore environment using a wireline, by the generation of a waveshape, for representing the digital data, that is not dependent on the alternate data states being represented by alternate voltage levels, but rather by representing each data bit leading transition edge by a high frequency pulse, thus preserving the phase relationship so that data can be recovered.